Motion in Polyethylene. II. Vibrations in Crystalline Polyethylene

Abstract

Using the specific heats of completely crystalline polyethylene as a basis, the vibrational spectrum is discussed. The vibrational spectrum is found to consist of three completely separate parts. (A) The high‐frequency CH₂‐stretching vibrations between 2850 and 2930 cm^—1 which contribute little to the specific heat below 350°K. (B) The low‐frequency optical vibrations between 720 and 1480 cm^—1 which contribute above 150°K increasingly to the specific heat. (C) The acoustical vibrations which stretch from zero to approximately 500 cm^—1 and are at 260°K to 90% excited. A and B are adequately known from infrared and Raman experiments. C can be approximated by a Tarasov‐type treatment leading to θ temperatures of 123° and 540° for the three‐dimensional and one‐dimensional parts, respectively. More detailed spectra are discussed, and a semiempirical best fit is given. The one‐dimensional Debye function necessary for this discussion has been computed to five places and is tabulated in steps of hν/kT=0.01.